1. Field of the Invention
The present invention relates to a fault detector for a vehicle safety system and more particularly to an inexpensive and highly reliable fault detector for a trigger resistor used in an air bag system.
2. Description of the Prior Art
A common air bag for a vehicle uses a trigger heater called a "squib", which is a resistor having a relatively small resistance value and enables the air bag to be instantaneously inflated at the moment of vehicle collision. Such a trigger resistor, if failed, would put passengers' lives in danger and therefore is constantly monitored for any anomaly in the resistance.
FIG. 1 shows a circuitry of a conventional fault detector used in a air bag system as disclosed in the Japanese Patent Publication No. Showa 61-57219.
In FIG. 1, element 1 represents a vehicle-mounted battery or a direct current power source, and an ignition switch 2 is connected to the dc power source 1 for starting engine. Element 3 is an acceleration sensor (hereinafter called a G sensor) connected to the dc power source 1 through the ignition switch 2 and consists of a normally open contact 31 and a resistor 32 connected in parallel with each other. Element 4 is an air bag activating squib or resistor to be monitored. The squib resistor 4 is connected at a junction A to the G sensor 3. The squib resistor 4 forms a first series circuit along with the resistor 32 of the G sensor 3. Another G sensor 5 is connected at a junction C to the squib resistor 4 and, like the first G sensor 3, consists of a normally open contact 51 and a resistor 52 connected in parallel with each other. The other end of the G sensor 5 is grounded.
Circuit 6 is a fault detecting circuit which is connected across the squib resistor 4 and also to the ends of the G sensors 3 and 5 for detecting a fault of the squib resistor 4. The fault detecting circuit 6 consists of a dc differential amplifier circuit 7 connected to junctions A and C and a comparator circuit 8 connected to an output terminal of the differential amplifier circuit 7. The differential amplifier circuit 7 consists of resistors 71 to 74 that determine an amplification factor and an operational amplifier 75, whereby the resistor 71 is interposed between the junction A and a non-inverted input terminal of the operational amplifier 75, the resistor 72 is between the ground and the non-inverted input terminal of the operational amplifier 75, the resistor 73 is between the junction C and an inverted input terminal of the operational amplifier 75, and the resistor 74 is between an output terminal of the operational amplifier 75 and the inverted input terminal.
The comparator circuit 8 consists of resistors 81 to 83 connected in series for dividing the voltage of the dc power source 1 and thereby providing reference voltages, an operational amplifier 84 a non-inverted input terminal of which is connected to a junction of resistors 81 and 82 whilst an inverted input terminal of which is connected to the output terminal of the operational amplifier 75, an operational amplifier 85 an inverted input terminal of which is connected to a junction of resistors 82 and 83 whilst a non-inverted input terminal of which is connected to the output terminal of the operational amplifier 75, and an AND gate 86 for multiplying the outputs of the operational amplifiers 84 and 85.
Element 9 is an alarm lamp connected to the output terminal of the comparator circuit 8 or to the output terminal of the AND gate 86.
Now, the operation of the prior art fault detector will be described.
When the ignition switch 2 is closed to start a vehicle engine, the G sensors 3 and 5, the squib resistor 4 and the fault detecting circuit 6 are supplied power from the dc power source 1. Since the normally open contacts 31 and 51 are open, a voltage is generated across the squib resistor 4 which is equal to a voltage V1 of the dc source 1 divided by the resistors 32 and 52 and the squib resistor 4.
The resistors 32 and 52 have resistance values R3 and R5 of more than several hundred ohm while the squib resistor 4 has a resistance value R4 of only several ohm. The supply voltage V1 is about 12 V, so the voltage difference V.sub.AC between the junctions A and C is several tens of mV. for example, if we assume: EQU R.sub.3 =R.sub.5 =1k.OMEGA. EQU R.sub.4 =2.OMEGA.
then, the voltage V.sub.AC across the squib resistor 4 is: ##EQU1## if we suppose the G sensor 3 is short-circuited, we get: ##EQU2## if the squib resistor 4 is also short-circuited, we get: EQU V.sub.AC =0V
To make it possible to decide whether or not there is any fault according to the voltage which varies in a range of 0-10's mV requires that the resistors 71 to 74 be adjusted so as to set the amplification factor of the differential amplifier 75 to approximately 100. As a result, the output voltage V7 of the differential amplifier circuit 7 is normally 1.2 V, however, in case of a fault, it will be 2.4 V when the G sensor 3 or 5 is short-circuited, while it will be 0 V when the squib resistor 4 is short-circuited.
Hence, the resistors 81 to 83 in the comparator circuit 8 are adjusted in such a way that, outputs of the operational amplifiers 84 and 85 become high "H" levels when the output voltage V7 of the differential amplifier circuit 7 is normal (V7=1.2 V), an output of the operational amplifier 84 becomes low "L" level when the G sensor 3 or 5 is short-circuited (V7=2.4 V), and an output of the operational amplifier 85 becomes low "L" level when the squib resistor 4 is short circuited (V7=0 V).
With this resistor setting, an output of the AND gate 86 goes to a high level in a normal condition for turning the alarm lamp 9 OFF, whilst the output of the AND gate goes to a low level in an abnormal condition for turning the alarm lamp 9 ON, thus alerting the driver to the abnormal condition.
When the G sensors 3 and 5 and the squib resistor 4 are in a normal condition, if a vehicle accident takes place, the normally open contacts 31 and 51 will be closed to heat the squib resistor 4, activating the air bag system to protect the driver from injury.
Since the prior art fault detector, as mentioned above, detects a voltage change resulting from a fault, it is necessary to set the amplification degree of the dc differential amplifier circuit 7 at around 100. This high amplification factor makes the fault detector susceptible to noise. Moreover, with use of dc differential amplification, an error is easily introduced by the influence of the input offset voltage of the amplifier circuit. This requires the use of high-precision amplifying devices as well as fine adjustments in the process of manufacturing, which in turn results in poor reliability and increased production cost.
It is therefore an object of this invention to overcome the aforementioned problems and to provide an inexpensive and highly reliable fault detector for an air bag in a vehicle safety system.